1. Field of the Invention
The present invention relates to a stacked electro-mechanical energy conversion element such as a piezoelectric element and a method of manufacturing the stacked electro-mechanical energy conversion element, and more particularly to a through-hole that forms a connection between the respective layers of the stacked electro-mechanical energy conversion element.
2. Related Background Art
Up to now, a piezoelectric material having an electro-mechanical energy conversion function is employed as various piezoelectric elements or piezoelectric devices. As a recent tendency, those piezoelectric elements and piezoelectric devices take not a single plate but a structure in which a large number of thin sheets are superimposed on each other so as to be stacked.
One main reason is that a large deformation or distortion or a large force can be obtained by a low supply voltage in a stacked piezoelectric element as compared with a single plate-like piezoelectric element. Also, another reason is that a sheet molding method or a manufacturing method for stacking have been spread, the thickness of each layer is thinned, and a stacked piezoelectric element and piezoelectric device which are downsized and high in performance are readily manufactured.
For example, a stacked piezoelectric element for a vibration wave motor which is an example of a vibration wave driving device has been disclosed by U.S. Pat. Nos. 6,046,526 and 5,770,916. The stacked piezoelectric element is not limited to a vibration wave driving device but employed for various devices such as a vibration gyro or a piezoelectric transformer.
The stacked electro-mechanical energy conversion element is of the structure in which a plurality of laminate materials each having an electro-mechanical energy conversion function on which an electrode region is formed are superimposed on each other. As a representative example, there is a structure in which a plurality of piezoelectric ceramics layers (hereinafter referred to as “piezoelectric layer”) on a surface of which an electrode layer formed of an electrode material (hereinafter referred to as “inner electrode”) is provided are stacked on each other.
As disclosed in U.S. Pat. Nos. 6,046,526 and 5,770,916, there is a structure that uses through-holes (or called “via holes”) formed by embedding an electrically conductive material (electrode material) in holes defined in the piezoelectric layer as interlayer wirings for connecting a plurality of stacked inner electrodes.
FIG. 10 shows a stacked piezoelectric element disclosed in U.S. Pat. No. 5,770,916, in which a stacked piezoelectric element 71 has a through-hole 76 for connecting the inner electrodes.
In FIG. 10, the stacked piezoelectric element 71 is of a columnar shape in the center of which a through-hole is formed, in which through-holes 76 indicated by black circles are formed at an inner diameter side. The through-holes 76 render the respective stacked piezoelectric layers conductive. On the surfaces of two kinds of piezoelectric layers 72 and 73 that constitute the stacked piezoelectric element 71 are formed an inner electrode (or called “electrode pattern”) 74 divided into four electrode portions (74-1, 74-2, 74-3, 74-4) and an inner electrode 75 divided into four electrode portions (75-1, 75-2, 75-3, 75-4). The piezoelectric layers 72 and 73 are alternately stacked on each other on a second layer to a twenty-fifth layer except for a first layer that is an upper surface layer.
Eight through-holes 76 indicated by black circles are formed at the inner diameter sides of the respective piezoelectric layers in the figure. Among them, four through-holes 76 render the electrode portions positioned at an inphase of the second layer, the fourth layer, the sixth layer, . . . the 24th layer (piezoelectric layer 72) conductive to each other.
Also, the remaining four through-holes 76 render the electrode portions positioned at an inphase of the third layer, the fifth layer, the seventh layer, . . . the 25th layer (piezoelectric layer 73) conductive to each other. Only the 25th layer is different in structure from other layers in that no through-holes are formed in the 25th layer because no layer is disposed below the 25th layer.
Also, the through-holes that are rendered conductive to the inner electrode 74 of the piezoelectric layer 72 are not rendered conductive to the inner electrode 75 of the piezoelectric layer 73 whereas the through-holes that are rendered conductive to the inner electrode 75 of the piezoelectric layer 73 are not rendered conductive to the inner electrode 74 of the piezoelectric layer 72.
Then, those eight through-holes 76 are rendered nonconductive to each other, and the end portions of the through-holes 76 are exposed on the surface of the stacked piezoelectric element 71 to form a surface electrode 77. Also, the respective inner electrodes 74 and 75 are not formed up to the edges of the outer diameter and the inner diameter, and non-electrode regions are formed at the edges of the outer diameter and the inner diameter.
The stacked piezoelectric element 71 thus structured as shown in FIG. 10 is subjected to the following polarizing process.
In the respective piezoelectric layers 73 that structure the third layer, the fifth layer, the seventh layer, . . . the 25th layer, the electrode portions having a positional relationship of 180 degrees have polarized polarities different from each other so that the electrode portions 75-1 and 75-3 are polarized to +(plus) and −(minus), respectively, and the electrode portions 75-2 and 75-4 are polarized to +(plus) and −(minus), respectively.
As shown in FIG. 10, it is assumed that the electrode portions 75-1 and 75-3 are A+ and A− respectively, the electrode portions 75-2 and 75-4 are B+ and B− respectively, and those electrode portions are phase A and phase B, respectively. Also, it is assumed that the electrode portions 74-1 and 74-3 of the piezoelectric layer 72 which face the electrode portions 75-1 and 75-3 are a phase AG, the electrode portions 74-2 and 74-4 of the piezoelectric layer 72 which face the electrode portions 75-2 and 75-4 are a phase BG, and those electrode portions are electrically grounded.
FIG. 11 is a cross-sectional view showing a bar-shaped vibration wave motor 140 that functions as a vibration wave driving device where the stacked piezoelectric element 71 are incorporated in a vibration member 141. The stacked piezoelectric element 71 and a wiring board 149 that comes in direct contact with the stacked piezoelectric element 71 are disposed between metal parts 142 and 143 which are elastic members of the vibration member 141 are fixed by fastening a bolt 144. The eight surface electrodes of the stacked piezoelectric element 71 and the electrode pattern of the wiring substrate 149 are electrically connected to each other.
The drive principle of the bar-shaped vibration wave motor 140 is as follows: two bending vibrations that are orthogonal to each other are produced in the vibration member 141 in which the stacked piezoelectric element 71 is incorporated, and a rotor 147 that comes in pressure contact with the vibration member 141 is driven by a frictional force. The rotor 147 is brought in pressure contact with the metal part 142 though a spring 145 and a spring support member 146.
The phase AG and the phase BG which face the phase A and the phase B are grounded, and a high frequency voltage that is substantially identical with a natural vibration frequency is applied to the phase A. Also, a high frequency voltage of the same vibration frequency as that of the phase A but electrically different in phase from the phase A by 90 degrees is applied to the phase B that is spatially different in phase from the phase A by 90 degrees, and drive vibrations are obtained by synthesizing those two bending vibrations produced in the vibration member 141.
Then, the rotor 147 that is in pressure contact with one surface of the metal part 142 is frictionally driven by the drive vibrations generated in the vibration member 141, and a drive force is outputted from the gear 148 that functions as an output member that rotates integrally with the rotor 147.
It is necessary to notch the inner electrode 75 and dispose an insulating portion (non-electrode region 79 in FIG. 10) in the periphery of the through-hole so that the through-hole that is rendered conductive to the inner electrode 74 of the piezoelectric layer 72 is not rendered conductive to the inner electrode 75 of the piezoelectric layer 73. The same is applied to the piezoelectric layer 72.
When the outer diameter of the stacked piezoelectric element is made small as the through-holes remain at the inner diameter side as in the stacked piezoelectric element shown in FIG. 10, because the circular insulating portion is disposed in the periphery of the through-hole, it is difficult to satisfactorily broaden a region of a piezoelectric active portion (a portion interposed between the inner electrode 74 and the inner electrode 75) necessary to drive the vibration wave motor. For that reason, there is a limit of downsizing the stacked piezoelectric element 71.